France's Constitutional Council ruled Tuesday that a French law concerning the mass killings of Armenians a century ago violates the country's constitution.

French President Nicolas Sarkozy, who had personally backed the law, immediately said he would ask the government to prepare a new bill taking into account the council's ruling.

The law passed by France's parliament in December makes it a crime to deny that the killings of some 1.5 million Armenians by Ottoman Turks in 1915 constituted genocide. The council ruled the law would violate freedom of expression and speech, which are guaranteed by the French constitution.

Turkey, which says there was no systematic campaign against Armenians, has strongly opposed the French law.

The head of a French Armenian organization, meanwhile, sharply criticized the ruling, saying it was the result of Turkish lobbying.

Relations between France and Turkey have suffered since the law's passage, with Turkey suspending its military and economic cooperation with France after the lower house approval of the measure in December. The French Senate gave the law the green light in late January.